Electrical resistivity methods involve the measurement of the apparent resistivity of soils and rock as a function of depth or position. The resistivity of soils is a complicated function of porosity, permeability, ionic content of the pore fluids, and clay mineralization. The most common electrical methods used in hydrogeologic and environmental investigations are vertical electrical soundings (resistivity soundings) and resistivity profiling
During resistivity surveys, current is injected into the earth through a pair of current electrodes, and the potential difference is measured between a pair of potential electrodes. The current and potential electrodes are generally arranged in a linear array. Common arrays include the dipole-dipole array, pole-pole array, Schlumberger array, and the Wenner array. The apparent resistivity is the bulk average resistivity of all soils and rock influencing the flow of current. It is calculated by dividing the measured potential difference by the input current, and multiplying by a geometric factor (specific to the array being used and electrode spacing).
In resistivity soundings, the distance between the current electrodes or the distance between the current and potential dipoles is expanded in a regular manner between readings, thus yielding information of the electrical properties of soils from deeper and deeper depths. Models of the variation of resistivity with depth can be obtained using model curves or forward and inverse modeling computer programs.
GEOVision uses resistivity soundings to:
- Characterize subsurface hydrogeology
- Determine depth to bedrock/overburden thickness
- Determine depth to groundwater
- Map stratigraphy
- Map clay aquitards
- Map salt-water intrusion
- Map vertical extent of certain types of soil and groundwater contamination
- Estimate landfill thickness
In resistivity profiling, the electrode spacing is fixed, and measurements are taken at successive intervals along a profile. Data are generally presented as profiles or contour maps and interpreted qualitatively.
Resistivity profiling is used to:
- Map faults
- Map lateral extent of conductive contaminant plumes
- Locate voids
- Map heavy metals soil contamination
- Delineate disposal areas
- Map paleochannels
- Explore for sand and gravel
- Map archaeological sites